An extensive body of evidence has unequivocally demonstrated that some degree of immune response against cancer exists in humans and animals. In cancer patients, cellular components of the immune system are able to recognize antigens expressed by tumor cells, such as differentiation of oncofetal antigens or mutated gene products (S. Rosenberg, Nature, 411:380-4 (2001); P. van der Bruggen et al., Immunological Rev., 188:51-64 (2002)). A number of clinical studies have shown that tumor-infiltrating lymphocytes have favorable prognostic significance (E. Halapi, Med. Oncol., 15(4):203-11 (1998); Y. Naito et al., Cancer Res., 58(16):3491-4 (1998); L. Zhang et al., N.E. J. Med., 348(3):203-13 (2003)). Furthermore, treatment with immunomodulators, such as cytokines or bacterial products, cancer vaccines, or adoptive immunotherapy has led to tumor regression in a number of patients (S. Rosenberg, Cancer J. Sci. Am. 6(S):2 (2000); P. Bassi, Surg. Oncol., 11(1-2):77-83 (2002); S. Antonia et al., Current Opinion in Immunol., 16:130-6 (2004)). Despite these responses, immunity against cancer frequently fails to effectively eliminate tumor cells. The causes for this failure can be grouped into three major categories: (i) impaired tumor recognition by immune cells, either by variable expression of tumor antigens or reduced expression of class I major histocompatibility complex (MHC); (ii) immunosuppressive tumor microenvironment, as a result of secretion of inhibitory cytokines by tumor cells (e.g., TGF-β); and (iii) poor tumor immunogenicity due to the lack of expression of co-stimulatory molecules on tumor cells, which results in the inability of the tumor cells to effectively stimulate T-cells. Advances in our understanding of the requirements for tumor antigen recognition and immune effector function indicate that a potential strategy to enhance an anti-tumor immune response is to provide co-stimulation through an auxiliary molecule. Tumor antigen-specific T-cells require costimulation to initiate and maintain effector functions. Thus, therapies that target costimulatory molecules can be applied to modulate and enhance the immune response to tumors.
The current model for T-cell activation postulates that naive T-cells require two signals for full activation: (i) a signal provided through the binding of processed antigens presented to the T-cell receptor by major histocompatibility complex (MHC) class I molecules; and (ii) an additional signal provided by the interaction of co-stimulatory molecules on the surface of T-cells and their ligands on antigen presenting cells. Recognition of an antigen by a naive T-cell is insufficient in itself to trigger T-cell activation. Without a co-stimulatory signal, T-cells may be eliminated either by death or by induction of anergy. Signaling through the CD28 costimulatory molecule appears to be key for the initiation of T-cell responses. However, CD137 (4-1BB) signaling has been shown to be primordial for the maintenance and expansion of the immune response to antigens, as well as, for the generation of memory T-cells.
CD137 (4-1BB) is a member of the tumor necrosis receptor (TNF-R) gene family, which includes proteins involved in regulation of cell proliferation, differentiation, and programmed cell death. CD137 is a 30 kDa type I membrane glycoprotein expressed as a 55 kDa homodimer. The receptor was initially described in mice (B. Kwon et al., P.N.A.S. USA, 86:1963-7 (1989)), and later identified in humans (M. Alderson et al., Eur. J. Immunol., 24: 2219-27 (1994); Z. Zhou et al., Immunol. Lett., 45:67 (1995)) (See, also, Published PCT Applications WO95/07984 and WO96/29348, and U.S. Pat. No. 6,569,997, hereby incorporated by reference (See, SEQ ID NO:2.)). The human and murine forms of CD137 are 60% identical at the amino acid level. Conserved sequences occur in the cytoplasmic domain, as well as 5 other regions of the molecule, indicating that these residues might be important for function of the CD137 molecule (Z. Zhou et al., Immunol. Lett., 45:67 (1995)). Expression of CD137 has been shown to be predominantly on cells of lymphoid lineage such as activated T-cells, activated Natural Killer (NK) cells, NKT-cells, CD4CD25 regulatory T-cells, and also on activated thymocytes, and intraepithelial lymphocytes. In addition, CD137 has also been shown to be expressed on cells of myeloid origin like dendritic cells, monocytes, neutrophils, and eosinophils. Even though CD137 expression is mainly restricted to immune/inflammatory cells, there have been reports describing its expression on endothelial cells associated with a small number of tissues from inflammatory sites and tumors.
Functional activities of CD137 on T-cells have been amply characterized. Signaling through CD137 in the presence of suboptimal doses of anti-CD3 has been demonstrated to induce T-cell proliferation and cytokine synthesis (mainly IFN-γ), and to inhibit activated cell death. These effects have been observed with both murine and human T-cells (W. Shuford et al., J. Exp. Med., 186(1):47-55 (1997); D. Vinay et al., Semin. Immunol., 10(6):481-9 (1998); D. Laderach et al., Int. Immunol., 14(10): 1155-67 (2002)). In both humans and mice, co-stimulation enhances effector functions, such as IFN-γ production and cytotoxicity, by augmenting the numbers of antigen-specific and effector CD8+ T-cells. In the absence of anti-CD3 signaling, stimulation through CD137 does not alter T-cell function, indicating that CD137 is a co-stimulatory molecule.
The physiological events observed following CD137 stimulation on T-cells are mediated by NF-κB and PI3K/ERK1/2 signals with separate physiological functions. NF-κB signals trigger expression of Bcl-XL, an anti-apopotic molecule, thus resulting in increased survival, whereas PI3K and ERK1/2 signals are specifically responsible for CD137-mediated cell cycle progression (H. Lee et al., J. Immunol., 169(9):4882-8 (2002)). The effect of CD137 activation on the inhibition of activation-induced cell death was shown in vitro by Hurtado et al. (J. Hurtado et al., J. Immunol., 158(6):2600-9 (1997)), and in an in vivo system in which anti-CD137 monoclonal antibodies (mabs) were shown to produce long-term survival of superantigen-activated CD8+ T-cells by preventing clonal deletion (C. Takahashi et al., J. Immunol., 162:5037 (1999)). Later, two reports demonstrated, under different experimental conditions, that the CD137 signal regulated both clonal expansion and survival of CD8+ T-cells (D. Cooper et al., Eur. J. Immunol., 32(2):521-9 (2002); M. Maus et al., Nat. Biotechnol., 20:143 (2002)). Reduced apoptosis observed after co-stimulation correlated with increased levels of Bcl-XL in CD8+ T-cells, while Bcl-2 expression remained unchanged. Up-regulation of the anti-apoptotic genes Bcl-XL and bfl-1 via 4-1BB was shown to be mediated by NF-κB activation, since PDTC, an NF-κB-specific blocker, inhibited 4-1BB-mediated up-regulation of Bcl-XL (H. Lee et al., J. Immunol., 169(9):4882-8 (2002)). On the other hand, clonal expansion of activated T-cells appears to be mediated by increased expression of cyclins D2, D3, and E, and down-regulation of the p27kip1 protein. This effect occurs in both an IL-2 dependent and independent fashion (H. Lee et al., J. Immunol., 169(9):4882-8 (2002)).
Altogether, CD137 stimulation results in enhanced expansion, survival, and effector functions of newly primed CD8+ T-cells, acting, in part, directly on these cells. Both CD4+ and CD8+ T-cells have been shown to respond to CD137 stimulation, however, it appears that enhancement of T-cell function is greater in CD8+ cells (W. Shuford et al., J. Exp. Med., 186(1):47-55 (1997); I. Gramaglia et al., Eur. J. Immunol., 30(2):392-402 (2000); C. Takahashi et al., J. Immunol., 162:5037 (1999)). Based on the critical role of CD137 stimulation in CD8+ T-cell function and survival, manipulation of the CD137/CD137L system provides a plausible approach for the treatment of tumors and viral pathogens.
Recently, the constitutive expression of CD137 on freshly isolated dendritic cells (DCs) was demonstrated in mice (R. Wilcox et al., J. Immunol., 169(8):4230-6 (2002); T. Futagawa et al., Int. Immunol., 14(3):275-86 (2002)) and humans (S. Pauly et al., J. Leukoc. Biol. 72(1):35-42 (2002)). These reports showed that stimulation of CD137 on DCs resulted in secretion of IL-6 and IL-12, and, more importantly, it enhanced DC ability to stimulate T-cell responses to alloantigens. Furthermore, Pan et al. demonstrated that CD137 signaling in DCs resulted in upregulation of MHC Class I and costimulatory molecules, and produced an increased ability of DCs to infiltrate tumors (P. Pan et al., J. Immunol., 172(8):4779-89 (2004)). Therefore, CD137 costimulation on DCs appears to be a novel pathway for proliferation, maturation, and migration of DCs.
Activated Natural Killer (NK) cells express CD137 following stimulation with cytokines (I. Melero et al., Cell Immunol., 190(2):167-72 (1998); R. Wilcox et al., J. Immunol., 169(8):4230-6 (2002)). Several reports demonstrated that NK cells appear to be critical for the modulation of the antitumor immune response induced by agonistic CD137 antibodies ((I. Melero et al., Cell Immunol., 190(2):167-72 (1998); R. Miller et al., J. Immunol., 169(4):1792-800 (2002); R. Wilcox et al., J. Immunol., 169(8):4230-6 (2002)). Depletion of NK cells significantly reduces the antitumor activity of anti-CD137 mabs. Ligation of CD137 on NK cells induces proliferation and IFN-γ secretion, but does not affect their cytolytic activity. Notably, in vitro, CD137-stimulated NK cells presented an immunoregulatory or “helper” activity for CD8+ cytolytic T-cells resulting in expansion of activated T-cells. Therefore, CD137 signaling on NK cells may modulate innate immunity to tumors.
A paradoxical effect has been described for CD137 stimulation in that agonistic CD137 antibodies can induce suppression of the humoral responses to T-cell antigens in primates and mouse models (H. Hong et al., J. Immunother., 23(6):613-21 (2000); R. Mittler et al., J. Exp. Med., 190(10):1535-40 (1999)). Notably, CD137 agonistic antibodies were shown to produce significant amelioration of the symptoms associated with antibody dependent autoimmune diseases such as systemic lupus erythematosus and experimental autoimmune encephalomyelitis (J. Foell et al., N.Y. Acad. Sci., 987:230-5 (2003); Y. Sun et al., Nat. Med., 8(12):1405-13 (2002)). Recently, Seo et al. demonstrated that, in a mouse model of rheumatoid arthritis, treatment with an agonistic anti-CD137 antibody prevented the development of the disease, and remarkably, blocked disease progression (S. K. Seo et al., Nat. Med. 10;1099-94 (2004)). The mechanism responsible for this effect has not been well defined, but in the model of rheumatoid arthritis it was shown that treatment with a CD137 agonistic antibody resulted in the expansion of IFN-γ-producing CD11C-CD8+ T cells. IFN-γ in turn stimulated dendritic cells to produce indolamine-2,3-dioxygenase (IDO), which exerts immuno-suppressive activities. It has also been postulated that CD137 signaling on antigen-activated CD4+ T-cells results in induction of IFN-γ secretion which activates macrophages. Activated macrophages can in turn produce death signals for B cells. Continuous signaling through CD137 on CD4+ T-cells may subsequently induce activation-induced cell death (AICD) of these CD4+ activated T-cells. Therefore, by eliminating antigen-activated T-cells and B cells, a reduced antibody response is observed and, consequently, a dramatic reduction of Th2-mediated inflammatory diseases is observed (B. Kwon et al., J. Immunol., 168(11):5483-90 (2002)). These studies suggest a role for the use of agonistic CD137 antibodies for the treatment of inflammatory or autoimmune diseases, without inducing a general suppression of the immune system.
The natural ligand for CD137, CD137 ligand (CD137L), a 34 kDa glycoprotein member of the TNF superfamily, is detected mainly on activated antigen-presenting cells (APC), such as B cells, macrophages, dendritic cells, and also on murine B-cell lymphomas, activated T-cells, and human carcinoma lines of epithelial origin (R. Goodwin et al., Eur. J. Immunol., 23(10):2631-41 (1993); Z. Zhou et al., Immunol. Lett., 45:67 (1995); H. Salih et al., J. Immunol., 165(5):2903-10 (2000)). Human CD137L shares 36% homology with its murine counterpart (M. Alderson et al., Eur. J. Immunol., 24: 2219-27 (1994)).
In addition to delivering signals to CD137-expressing cells, binding of CD137 to CD137L initiates a bidirectional signal resulting in functional effects on CD137L-expressing cells. Langstein et al. demonstrated that binding of CD137-Ig fusion protein to CD137L on activated monocytes induced the production of IL-6, IL-8, and TNF-α, upregulated ICAM, and inhibited IL-10, resulting in increased adherence (J. Langstein et al., J. Immunol., 160(5):2488-94 (1998)). In addition, proliferation of monocytes was demonstrated along with a higher rate of apoptosis (J. Langstein et al., J. Leukoc. Biol., 65(6):829-33 (1999)). These observations were confirmed by the studies of Ju et al. (S. Ju et al., Hybrid Hybridomics, 22(5):333-8 (2003)), which showed that a functional anti-CD137L antibody induced a high rate of proliferation of peripheral blood monocytes. Blocking the ligand resulted in inhibition of T-cell activation. In addition, soluble CD137L was found in the serum of patients with rheumatoid arthritis and hematological malignancies (H. Salih et al., J. Immunol., 167(7):4059-66 (2001)). Thus, the interaction of CD137 with CD137L influences and produces functional effects on T-cells and APC.
In another important aspect of T-cell function, it has been demonstrated that agonistic anti-CD137 antibodies rescued T-cell responses to protein antigens in aged mice. It has been well documented that an age-related decline in the immune response to antigens occurs, a process known as immunosenescence (R. Miller, Science, 273:70-4 (1996); R. Miller, Vaccine, 18:1654-60 (2000); F. Hakim et al., Curr. Opinion Immunol., 16:151-156 (2004)). This phenomenon appears to be due to alterations in the equilibrium between the extent of cellular expansion and cellular survival or death. Bansal-Pakala et al. tested the hypothesis that secondary costimulation through CD137 can be used to enhance T-cell responses in situations where T-cells do not receive sufficient stimulation, due to either reduced expression of CD3 or CD28, or reduced quality of signals. Their studies showed that aged mice had a deficient in vitro recall response compared to young mice (P. Bansal-Pakala et al., J. Immunol., 169(9):5005-9 (2002)). However, when aged mice were treated with anti-CD137 mabs, the proliferative and cytokine responses of T-cells were identical to the responses observed in young mice. While the specific mechanism responsible for this effect was not elucidated, it was speculated that enhancing both the expression of anti-apoptotic molecules like Bcl-XL, and the promotion of IL-2 secretion in vivo may play a role in rescuing defective T-cell responses. These studies demonstrated the potential for agonistic anti-CD137 antibodies to rescue weak T-cell responses in elderly immuno-compromised individuals, and has profound implications for the use of anti-CD137 antibodies in cancer patients.
A role for CD137 targeted therapy in the treatment of cancer was suggested by in vivo efficacy studies in mice utilizing agonistic anti-murine CD137 monoclonal antibodies. In a paper by Melero et al., agonistic anti-mouse CD137 antibody produced cures in P815 mastocytoma tumors, and in the low immunogenic tumor model Ag104 (I. Melero et al., Nat. Med., 3(6):682-5 (1997)). The anti-tumor effect required both CD4+ and CD8+ T-cells and NK cells, since selective in vivo depletion of each subpopulation resulted in the reduction or complete loss of the anti-tumor effect. It was also demonstrated that a minimal induction of an immune response was necessary for anti-CD137 therapy to be effective. Several investigators have used anti-CD137 antibodies to demonstrate the viability of this approach for cancer therapy (J. Kim et al., Cancer Res., 61(5):2031-7 (2001); O. Martinet et al., Gene Ther., 9(12):786-92 (2002); R. Miller et al., J. Immunol., 169(4):1792-800 (2002); R. Wilcox et al., Cancer Res., 62(15):4413-8 (2002)).
In support of the anti-tumor efficacy data with agonistic CD137 antibodies, signals provided by CD137L have been shown to elicit CTL activity and anti-tumor responses (M. DeBenedette et al., J. Immunol., 163(9):4833-41 (1999); B. Guinn et al., J. Immunol., 162(8):5003-10 (1999)). Several reports demonstrated that gene transfer of CD137 ligand into murine carcinomas resulted in tumor rejection, demonstrating the requirement of costimulation in generating an efficient immune response (S. Mogi et al., Immunology, 101(4):541-7 (2000); I. Melero et al., Cell Immunol., 190(2):167-72 (1998); B. Guinn et al., J. Immunol., 162(8):5003-10 (1999)). Salih et al. reported the expression of CD137L in human carcinomas and human carcinoma cell lines (H. Salih et al., J. Immunol., 165(5):2903-10 (2000)), and demonstrated that tumors cells expressing the ligand were able to deliver a co-stimulatory signal to T-cells which resulted in the release of IFN-γ and IL-2, and that this effect correlated with the levels of CD137L on tumors. Whether expression of CD137L in human tumors could make these tumors more susceptible to agonistic CD137 antibodies is not known.
CD137L −/− mice have underscored the importance of the CD137/CD137L system in T-cell responses to both viruses and tumors (M. DeBenedette et al., J. Immunol., 163(9):4833-41 (1999); J. Tan et al., J. Immunol., 164(5):2320-5 (2000); B. Kwon et al., J. Immunol., 168(11):5483-90 (2002)). Studies using CD137- and CD137L-deficient mice have demonstrated the importance of CD137 costimulation in graft-vs-host disease, and anti-viral cytolytic T-cell responses. CD137-deficient mice had an enhanced proliferation of T-cells, but a reduction in cytokine production and cytotoxic T-cell activity (B. Kwon et al., J. Immunol., 168(11):5483-90 (2002); D. Vinay et al., Immunol. Cell Biol., 81(3):176-84 (2003)). More recently, it was shown that knockout mice (CD137−/−) had a higher frequency of tumor metastases (4-fold) compared to control mice. These data suggest that restoration of CD137 signaling by the use of agonistic anti-CD137 antibodies is a feasible approach for augmenting cellular immune responses to viral pathogens and cancers.
In addition to the data in mouse in vivo models which supports the involvement of CD137 signaling in antitumor immune responses, studies conducted in primary human tumor samples have confirmed the role of CD137 in generating effector T-cells. In patients with Ewing sarcoma, Zhang et al. showed that intratumoral effector T-cells presented the CD3+/CD8+/CD28−/CD137+ phenotype. Unexpectedly, coexistence of progressive tumor growth and anti-tumor immunity (effector T-cells) was observed. Ex vivo stimulation studies with patients' cells demonstrated that tumor-induced T-cell proliferation and activation required costimulation with CD137L. Stimulation of PBL with anti-CD3/CD137L, but not anti-CD3/anti-CD28, induced tumor lytic effectors. These studies provided further evidence that CD137 mediated costimulation could result in expansion of tumor reactive CTLs (H. Zhang et al., Cancer Biol. Ther., 2(5):579-86 (2003)). Furthermore, expression of CD137 was demonstrated in tumor infiltrating lymphocytes in hepatocellular carcinomas (HCC) (Y. Wan et al., World J. Gastroenterol, 10(2):195-9 (2004)). CD137 expression was detected in 19 out of 19 HCC by RT-PCR, and in 13/19 by immunofluorescence staining. Conversely, CD137 was not detected in the peripheral mononuclear cells of the same patients. Analyses conducted in healthy donor liver tissues failed to demonstrate expression of CD137. These studies did not attempt to correlate clinical disease with CD137 expression. Thus, studies conducted in Ewing sarcoma and hepatocellular carcinoma revealed the presence of TIL that express CD137, with concomitant disease progression. In Ewing sarcomas it was demonstrated that CD137+TILs were able to kill tumor cells ex-vivo suggesting that the CD137 pathway was intact in these patients, and that perhaps suppressive factors in the tumor microenvironment inhibited their function. Hence, it can be postulated that systemic administration of agonistic CD137 antibodies may provide the signal necessary for expansion of these effector T-cells.
In addition to its role in the development of immunity to cancer, experimental data supports the use of CD137 agonistic antibodies for the treatment of autoimmune and viral diseases (B. Kwon et al., Exp. Mol. Med., 35(1):8-16 (2003); H. Salih et al., J. Immunol., 167(7):4059-66 (2001); E. Kwon et al., P.N.A.S. USA, 96:15074-79 (1999); J. Foell et al., N.Y. Acad. Sci., 987:230-5 (2003); Y. Sun et al., Nat. Med., 8(12):1405-13 (2002) S. K. Seo et al, Nat. Med. 10;1099-94 (2004)).
Consequently, based on the roles of 4-1BB in modulating immune response, it would be desirable to produce anti-human 4-1BB antibodies with agonistic activities that could be used for the treatment or prevention of human diseases such as cancer, infectious diseases, and autoimmune diseases.